A typical dynamic electromechanical system has a source of electrical energy, a prime mover and a load. The prime mover is activated by the electrical energy applied to it to mechanically act on the load.
For example, two types of electromechanical systems include rotary DC motors or solenoids as the prime mover. In a rotary DC motor, the torque and angular velocity are linearly proportional to current and voltage, respectively. The system linearity simplifies servo control.
However, in the typical solenoid system, the source is unaware of the load. Variations in the load are not sensed by the source. The source is unable to alter the electrical energy applied to the prime mover in response to the variations in the load.
In order to perform real time servo control so that the source is aware of variations in the load, a model or algorithm must be formulated so that the feedback control to the source can be computed. However, such models and algorithms are generally non-linear and complex. Real time servo control is thus generally not possible. The amount of time required to compute the feedback control from non-linear and complex models and algorithms is much greater than the time in which variations in the load occur. Thus, real time servo control has been widely ignored in the art in solenoid systems, and the source has usually been designed to provide sufficient electrical energy for all expected load conditions. This practice usually has the result that the source is providing excess energy during most load conditions.
An example of such an electromechanical system is a solenoid activated cardiac assist pump of the type disclosed in U.S. patent application Ser. No. 211,210, filed Nov. 28, 1980. The pump disclosed therein has a solenoid, a pump bladder, and a pair of springs operatively coupled between the solenoid and bladder. The solenoid has a pair of C-shaped cores being disposed in a facing relationship with each other and normally biased apart from each other. Upon application of a current to the solenoid, such as by capacitive discharge, the cores are accelerated towards each other. The closing of the solenoid generates sufficient force in the springs to compress the bladder.
Since the springs are also used to bias the solenoid, the initial current must be sufficient to overcome the biasing spring force. Since the initial gap between the solenoid cores is unknown, and thus the amount of bias spring force is unknown, excessive current is applied to overcome such force. However, the successive current also results in excessive and undesirable impact energies when the solenoid cores strike against each other upon closure. In this device, the spring force profile and magnetic force profile were ideally matched in attempt to provide sufficient initial current to close the solenoid, and to balance the total spring and magnetic force to eliminate the impact energies. Because of the unknown initial gap, this approach also resulted in "misfires" when the gap was larger than predicted whereby the solenoid failed to close, or in excessive impact energies when the gap was smaller than predicted. Furthermore, when the solenoid reached closure, a snare pulse applied to the solenoid may have been required to assure latching.